1. Field of the Invention.
The present invention relates to radiography. In particular, the present invention is an improved scanning x-ray system useful in a wide range of radiographic systems.
2. Description of the Prior Art.
Radiographic systems typically include an x-ray tube which emits x-rays. These x-rays are directed through an object to be studied (such as a portion of the human body) and onto an x-ray film. The x-ray tube typically includes an anode and a cathode.
When a very high voltage is applied between the anode and cathode, an electron beam is directed from the cathode along an anode-cathode axis and onto a target portion of the anode. The small area of the target which the electrons strike is called the focal spot, and is the source of the x-rays emitted by the tube.
The impact of the electrons at the focal spot generates both x-rays and a significant amount of heat. This heat must be dissipated so that the target, and therefore the tube, is not destroyed.
The quality of a radiographic image depends on the size and configuration of the focal spot. The smaller the focal spot, the better the detail of the image. Ideally the focal spot should be a point source. However, since a large spot can tolerate more heat than a small one, reducing the focal spot to a point source is not feasible without reducing the instantaneous heat loading capacity to levels which would preclude use in a clinical radiographic system.
One common technique for reducing the effective size of the focal spot is by using the line focus principle and rotating the anode. The line focus principle uses a target which is at an angle with respect to a plane perpendicular to the anode-cathode axis. The electron stream is focused on a narrow rectangle on the target. When the rectangular focal spot is viewed from below the anode, the focal spot is essentially square, and the effective area of the focal spot is only a fraction of its actual area. The length of the projected focal spot can be decreased by making the target angle smaller. However, a small target angle compromises field coverage. For adequate coverage in angiography, x-ray tubes having an 11.degree. or 12.degree. target are presently being used. For standard radiography, the target angle has to be larger to allow the coverage of a 35.9.times.43.6 centimeter (14.times.17 inch) radiograph at 1.02 meters (40 inches).
Further increases in the capacity of the anode to withstand heat have been achieved with rotating anode tubes. In these tubes, the anode is disk shaped, and has a beveled target area near its end. The cathode is arranged to direct the electron stream against the beveled target area of the disk, while the disk rotates. The position of the focal spot remains fixed in space while the anode rotates, thereby increasing the effective area being exposed to the electron beam, while maintaining the focal spot at a much smaller area than is possible with a stationary anode.
Since the introduction of the high speed rotating anode x-ray tubes, little progress has been made in significantly increasing instantaneous heat tube loading. As a result, the effective focal spot size for standard radiographic systems has not been significantly decreased. There is a continuing need for radiographic systems which produce superior radiographs with high resolution, increased contrast, and less patient exposure than in the prior art standard radiographic systems.
Tomography is a special radiographic technique in which a distinct image of a selected plane through the object is produced, while images of structures that lie on opposite sides of the plane are blurred. The value of tomography in clinical practice is well established.
In most presently used tomographic systems the patient remains stationary, and the x-ray tube and film cassette are moved. In these types of systems, the tube is typically mounted on a long arm, and as it is moved there is unavoidable vibration. As a result, the apparent focal spot size increases due to the vibration, thereby producing geometric unsharpness.
It has been known for many years that tomographs (and in particular laminographs) can be obtained by moving the object and film cassette and keeping the radiographic tube stationary. This principle was introduced by Vallebona in "Una modalita di tecnica per la dissociazione radiografica delle ombre applicate allo studio del cranio", Radiol Med 17: 1090-1097, September 1930, and was perfected by Bozetti, who actually built such an apparatus. See "La realizzazione practica della stratigrafia", Radiol Med 22: 257-267, 1935. With Bozetti's laminogram, the patient rotated about a craniocaudal axis; and the radiographic plate moved synchronously. This apparatus, however, never became popular because structures lying transversely (such as the ribs) could not be sufficiently blurred.
Previously, tomography with a linear patient motion was not possible because of the limited field coverage of standard radiographic tubes. If the object and film are moved synchronously, the object would move out of the x-ray beam. This method was suggested by Heckmann in "Die Roentgenperspektive und ihre umwandlung durch eine neue Aufnahmetechnik", Fortschr Roentgenstr 60: 140, 139. Heckman used a stationary x-ray tube with the patient and cassette moving in opposite directions. With this technique, only a very small area of the body could be examined, and therefore the method was impractical.
Another form of tomography which has presently found use is pantomography, which is described by Pantero in "A new tomographic method for radiographing curved outer surfaces", Acta Radiol 32: 177, 1949. In this system, the patient and a curved x-ray film rotate in opposite directions to yield a true laminogram. This type of laminography is useful only for the examination of the jaw.
Conventional tomography, therefore, has the shortcoming of poor resolution due to vibration of the x-ray tube, poor contrast due to a high fog level, and restriction to cuts parallel to the patient table. It is desirable to improve further the quality and diagnostic yield of tomography, and to provide means by which tomograms can be made of organs difficult to image because they do not lie in a plane parallel to the patient table.
Still another radiographic technique is angiography-the study of blood vessels. In some cases it is necessary to study the arteries from the abdomen of a patient all the way down to his toes. This large field coverage has necessitated special angiographic systems. In many cases the x-ray tube in an angiographic system must be mounted a large distance from the patient table in order to achieve the desired field coverage. This has required an extremely high ceiling of the room within which the angiographic system is located. In some cases, an entire radiology department must be specially designed to accommodate a room with an extremely high ceiling in order to house the angiographic system.
There is a continuing need for an angiographic system which provides the necessary wide field coverage and yet can be housed in a conventional room, rather than requiring a room with an extremely high ceiling.